This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-38723, filed Feb. 17, 2003, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a Magnetic Random Access Memory (MRAM), and more particularly to a method of reading memory information by using reference cells.
2. Description of the Related Art
An MRAM is a device that stores xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d data by making use of a magnetoresistive effect. It has been developed as one of the promising universal memory devices featuring nonvolatility, high integration, high reliability, low power consumption, and high-speed operation.
As for the magnetoresistive effect, the following two are chiefly known: GMR (Giant Magneto Resistance) and TMR (Tunneling Magneto Resistance). Of these, GMT makes use of the fact that the resistance of a conductor sandwiched between two ferromagnetic layers varies with the directions of the spins in the upper and lower ferromagnetic layers. However, the MR ratio representing the rate of a change in the magnetic resistance is as low as about 10%. Therefore, in an MRAM using GMR, a stored-data read signal is so small that it is difficult to secure a read margin. For this reason, the MRAM has been considered to be unsuitable for practical applications.
On the other hand, TMR, which has a stacked structure composed of an insulating film sandwiched between two metals acting as ferromagnetic layers, makes use of a change in the magnetoresistance caused by the spin polarization tunnel effect. Specifically, when the directions of the spins in the upper and lower layers are parallel with each other, the tunnel probability between the two magnetic layers with the tunnel insulating film between them becomes maximum, with the result that the resistance becomes minimum. With the directions of the spins being nonparallel with each other, when the spin polarization tunnel probability becomes minimum, the resistance becomes maximum. To realize two such spin states, one of the two magnetic layers is normally fixed in the direction of its magnetization and is set so as not to be influenced by external magnetization. Generally, the layer whose magnetization is fixed in direction is called a pin layer. The other magnetic layer enables the direction of magnetization to be programmed so as to be parallel or nonparallel with the pin layer, depending on the direction of the applied magnetic field. This layer, which is generally called a free layer, has the function of storing information. In TMR, the MR ratio as the resistance change rate has exceeded 50%. Thus, TMR is becoming dominant in the development of MRAMs.
When an MRAM using TMR is written into, the direction of magnetization in the free layer is reversed. To do this, current larger than a specific value is caused to flow through a bit line and a word line passing through each memory in such a manner that they cross at right angles with each other. The magnitude of the resultant magnetic field generated by the current controls the direction of the magnetization in the free layer.
On the other hand, methods of reading the stored data from a memory cell includes a method of applying a voltage between the two magnetic layers of the TMR corresponding to the selected bit and reading the resistance from the current flowing through them and a method of causing a constant current through the selected TMR and reading the voltage generated between the two magnetic layers by the current.
Specifically, a self-reference reading method (or destructive reading method) has been proposed (as disclosed in U.S. Pat. No. 6,134,138). In this patent reference 1, an example of an MRAM using GMR has been described. The same reading method can be applied to TMR. This method enables the largest reading signal to be secured between xe2x80x9c1xe2x80x9d data items or xe2x80x9c0xe2x80x9d data items and therefore is a very effective method for securing a reading margin.
FIG. 14 is a flowchart for the conventional self-reference reading method. Although FIG. 14 shows only the sequence of reading one bit of data, any number of bits of data requested by the system are read in an actual memory. In the read sequence, a word line and a bit line are activated suitably, thereby reading the resistance of the selected cell. The result of the reading is held in a data buffer A (first reading) (S1). Then, data xe2x80x9c1xe2x80x9d is written into, for example, the selected cell (first writing) (S2). Thereafter, the resistance of the selected cell is read again. The information is held in a data buffer B (second reading) (S3). In this state, it is determined whether the value of the data buffer A is equal to that of the data buffer B (S4). By making use of the fact that data xe2x80x9c1xe2x80x9d has been written in the first write operation, when the value of the data buffer A is equal to that of the data buffer B, it is determined that the data read from the selected cell is xe2x80x9c1xe2x80x9d (S5). When the value of the data buffer A differs from that of the data buffer B, it is determined that the data read from the selected cell is xe2x80x9c0xe2x80x9d (S6).
In the sequence of FIG. 14, data xe2x80x9c1xe2x80x9d is always written in the selected cell, regardless of the stored information before the reading. That is, the data before the reading may have been destroyed. This is why it is called a destructive reading method. When it is determined that the data read from the selected cell is xe2x80x9c0xe2x80x9d, the data in the selected cell has been rewritten from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d in the first write operation. Therefore, it is necessary to write data xe2x80x9c0xe2x80x9d into the selected cell (second writing) (S7).
As described above, the self-reference reading method, or the destructive reading method, requires two read operations and two write operations at worst, that is, a total of four cycles, which makes high-speed access difficult. Furthermore, in the MRAM, the drawn current in a write operation is generally larger than in a read operation. As a result, two write operations in a read operation cause the power consumption of the chip itself to increase further, which makes it difficult to introduce MRAMs into the market for small portable information terminals and the like.
According to an aspect of the present invention, there is provided a magnetic random access memory device comprising: a memory cell array in which a plurality of memory cells producing a magnetoresistive effect are arranged in a matrix; a plurality of reference cells which are provided in a part of the memory cell array and store data to be referred to when the memory cells are read from; word lines arranged in each row of the memory cell array; bit lines arranged in each column of the memory cell array; a row decoder to selectively drive the word lines; a column decoder to select the bit lines; a sense amplifier to sense the data in the selected memory cell and the data in the selected reference cell; a first data buffer to hold the data outputted from the sense amplifier; a controller which reads the data from the selected memory cell and then inverts the data in the selected reference cell; a second data buffer to hold the data outputted from the sense amplifier according to the data in the selected memory cell and the inverted data in the reference cell; and a comparator to compare the output data of the first data buffer and that of the second data buffer and determine the data read from the selected cell.
According to another aspect of the present invention, there is provided a magnetic random access memory device comprising: word lines arranged in each row of a memory cell array; bit lines arranged in each column of the memory cell array; a row decoder to selectively drive the word lines; a column decoder to select the bit lines; a plurality of memory cells producing a magnetoresistive effect which are arranged at half of the intersections of the word lines and the bit lines; a plurality of reference cells which are provided in a part of the memory cell array, are selected by dedicated word lines, and store data to be referred to when the memory cells are read from; a sense amplifier to sense the data in the selected memory cell and the data in the selected reference cell; a first data buffer to hold the data outputted from the sense amplifier; a controller which reads the data from the selected memory cell and then inverts the data in the selected reference cell; a second data buffer to hold the data outputted from the sense amplifier according to the data in the selected memory cell and the inverted data in the reference cell; and a comparator to compare the output data of the first data buffer and that of the second data buffer and determine the data read from the selected cell.
According to a further aspect of the present invention, there is provided a magnetic random access memory reading method comprising: sensing the difference between the data read from a selected cell and the data read from a reference cell in a first read operation; writing the reverse of the data read from the selected cell in the first read operation into the reference cell; sensing the difference between the data read from the selected cell and the data read from the reference cell in a second read operation; and determining the data stored in the selected cell from whether the difference between the data sensed in the first read operation is larger or smaller than the difference between the data sensed in the second read operation.